Shear-Viscosity to Entropy-Density Ratio from Giant Dipole Resonances in Hot Nuclei

TL;DR

This paper uses experimental data on giant dipole resonances in hot nuclei to calculate the shear viscosity to entropy density ratio, revealing it decreases with temperature and approaches a universal bound at 5 MeV.

Contribution

It introduces a novel method to determine the shear viscosity to entropy density ratio in hot nuclei directly from GDR data, combining experimental systematics with theoretical models.

Findings

$ ext{eta}/s$ decreases with increasing temperature T.

$ ext{eta}/s$ approaches $(1.3 - 4) imes rac{ ext{hbar}}{4 ext{pi}k_B}$ at T=5 MeV.

Results are consistent with almost model-independent estimations.

Abstract

The Green-Kubo relation and fluctuation-dissipation theorem are employed to calculate the shear viscosity $\eta$ of a finite hot nucleus directly from the width and energy of the giant dipole resonance (GDR) of this nucleus. The ratio $\eta/s$ of shear viscosity $\eta$ to entropy density $s$ is extracted from the experimental systematics of the GDR in copper, tin and lead isotopes at finite temperature $T$. These empirical results are then compared with the predictions by several independent models, as well as with almost model-independent estimations. Based on these results, it is concluded that the ratio $\eta/s$ in medium and heavy nuclei decreases with increasing temperature $T$ to reach $(1.3 - 4)\times\hbar/(4\pi k_{B})$ at $T=$ 5 MeV.